1. Field of the Invention
This invention relates to a high speed field effect transistor using high mobility of two-dimensional electron gas, more particularly to a depletion mode two-dimensional electron gas field effect transistor and the method for manufacturing the same.
2. Description of the Prior Art
The two-dimensional electron gas field effect transistor (hereinafter, referred to as 2-DEG.FET) attracts many engineer's attentions due to its high speed operability. The 2-DEG.FET uses high mobility of two-dimensional electron gas, that is, electrons confined in a potential well formed on the side of GaAs at the boundary between GaAs and impurity-doped AlGaAs. The electrons are supplied from the AlGaAs to the potential well to form the electron gas. Since such electron gas exhibits a property controlled by the scattering probability of two-dimensional electron gas, the electron gas has a very high mobility.
The 2-DEG.FET still has some drawbacks. One of them is a leakage current flowing between source and drain regions when the channel under the gate electrode is pinched off. The occurrence of such leakage current will now be explained.
The 2-DEG.FET is formed on a semi-insulator GaAs substrate on which a pure or N.sup.- -type GaAs layer and an impurity-doped N-type super lattice layer of GaAs and AlAs are consecutively formed. Source and drain regions are formed by alloying Au-Ge alloy from the surface of the super lattice layer. A gate electrode is formed on the super lattice layer between the source and drain regions by evaporating W-Si.
Under the side portions of the gate electrode, depletion layers cannot extend to fully pinch off the potential well in the surface region of the GaAs layer. Through the unpinched potential well portion, the leakage current flows between the source and drain regions. Therefore, the depletion mode 2-DEG.FET cannot be driven into a condition that the current flowing between source and drain regions becomes zero. This phenomenon results in lowering the resistance between source and drain regions, causing a deterioration of high speed operation.